The dynamics of the mobile phone market illustrate how much the pace of adoption has changed.

Only a few years ago, well-fewer than half of all mobile phones in the market were smartphones. But smartphones rapidly eclipsed those older “feature phones” – so that now only a very small percentage of cellphones in use today are of the feature phone variety.

Now, in just as little time we’re seeing smartphones go from boom to … well, not quite bust. In fewer than four years, the growth in smartphone sales has slowed from ~30% per year (in 2014) to just 4%.

That’s the definition of a “mature” market. But it also demonstrates just how successful the smartphone has been in penetrating all corners of the market.

Consider this: Market forecasting firm Ovum figures that by 2021, the smartphone will have claimed its position as the most popular consumer device of all time, when more than 5 billion of them are expected to be in use.

It’s part of a larger picture of connected smart devices in general, for which the total number in use is expected to double between now and 2021 – from an estimated 8 billion devices in 2016 to around 15 billion by then.

According to an evaluation conducted by research firm GfK, today only around 10% of consumers own either an Amazon Echo or Google Home device, but digital voice assistants are on the rise big-time. These interactive audio speakers offer a more “natural” way than smartphones or tablets to control smart home devices, with thousands of “skills” already perfected that allow them to interact with a large variety of apps.

There’s no question that home devices are the “next big thing,” but with their ubiquity, smartphones will continue to be the hub of the smart home for the foreseeable future. Let’s check back in another three or four years and see how the dynamics look then.